This invention relates in general to pressure vessels and more particularly to a layered head for a pressure vessel, with the head having machined surfaces, and to a method of producing such a head.
The typical pressure vessel has a cylindrical shell and heads that close the two ends of the shell. The shell and heads are manufactured as separate components and are thereafter joined together by welding to produce a unitary structure. To enable the heads to better withstand elevated pressures, they are usually dome-shaped.
It is generally recognized that pressure vessel walls composed of multiple layers are superior to thick solid walls in many respects. For example, the individual plates of a layered wall, generally speaking, having better metallurgical properties than thick solid walls, since they are subjected to greater rolling at the mill. As a consequence, a layered vessel is usually stronger than a solid wall vessel of equivalent wall thickness. Similarly, because of their better metallurgical properties, the individual layers of layered walls do not tend to laminate as sometimes occurs with solid walls. Also, in layered vessels it is possible to vary the metal alloy from layer to layer, thus enabling an expensive corrosion resistant liner to be used with less expensive surrounding layers. While a variety of thick steel plate clad with various corrosion resistant alloys is available from steel mills, it is expensive. Moreover, thin layers are relatively easy to shape, but this is not the case with the heavy steel plate used in solid wall vessels. Thus, layered walls can be manufactured in greater thicknesses than solid walls. Aside from that, the individual layers that comprise the walls of a layered vessel, upon being welded together, tend to shrink as the welds which join them solidifiy and cool, and this places the inner layers in a state of precompression. This is desirable since the elevated pressures within the vessel create tensile forces in the vessel walls. In contrast, solid wall vessels are normally heat treated to relieve them of stress concentrations, and therefore do not exist in a state of precompression.
Heretofore different procedures have been developed for fabricating cylindrical shells from multiple layers, one highly successful procedure being set forth in U.S. Pat. No. 3,478,784. Heads, by reason of their compound curvatures, are not so easily fabricated in multiple layers, and as a consequence most heads are still of the solid wall construction. Thus, to a large measure, the pressures to which present pressure vessels may be raised are set by the heads at their ends.